Field of the Invention
The invention relates to a method for optimising the speed of a twin-spool turbojet engine fan, and to a turbojet engine architecture for implementing this method.
Description of the Related Art
The field of the invention is that of the energy efficiency of two-flow, twin-spool or even three-spool turbojet engines. In a two-flow turbojet engine, the intake air flow separates beyond the fan into a secondary air flow, accelerated by the blades of the fan to form a supplementary thrust without passing through the combustion chamber, and a primary flow which can be compressed in order to be injected into the combustion chamber. In a twin-spool architecture, a low pressure (LP) turbine and a high pressure (HP) turbine are coupled respectively to coaxial LP and HP shafts suitable respectively for driving LP and HP compressors for compressing the primary flow, the fan also forming the first compression stage of the LP compressor.
To improve the propulsion energy efficiency, which translates into a reduction in the specific fuel consumption (SFC), the secondary flow must be able to generate the greatest possible thrust with, consequently, the greatest possible reduction in exhaust velocities of particles of air and waste gas. Three-flow turbojet engines have thus been developed. However, they are rarely used, as the increase in thrust obtained compared to a two-flow turbojet engine is too small given the additional volume which results from the presence of the third spool.
The efficiency can still be substantially improved, in particular when the engine is at cruising power, by the use of speed reducers, for example of the planetary gear train type.
The use of a planetary gear train is known for reducing the specific fuel consumption. This train is generally mounted between an LP turbine and an appropriate portion of LP compressors, as described in EP 1 933 017 or EP 1 931 290.
Such a planetary gear train can also serve as a speed accelerator for driving a fan and an LP supercharger in counter rotation from the LP shaft, as described in patent application WO 2008 105815. This application also provides for progressive engagement by meshing of the LP shaft.
In application WO2006059970, the planetary gear train makes it possible to optimise the distribution of power from the LP shaft between the fan and the LP compressor, depending on the flight conditions.
Furthermore, a magnetic or electromagnetic speed reducer has been used in EP 1 933 017 to transfer power and torque from the HP shaft to the LP shaft in order to drive electric generators at low speeds.